The invention relates to an exhaust gas system of an internal combustion engine, in particular for a motor vehicle having an exhaust gas conveying duct, at least one insert disposed in the exhaust gas conveying duct for cleaning the exhaust gas, an injection system disposed upstream of the insert in the flow path, and a turbulizer disposed between the injection system and the insert.
In order to clean exhaust gases, the exhaust gas systems for vehicles have inserts, for example, catalysts or diesel particulate filters with upstream catalysts, which filter the pollutants out of the exhaust gas. They have to be regenerated at regular intervals, because otherwise their flow resistance increases unduly fast or, more specifically, their efficiency decreases. This regeneration is generally achieved by burning off the exhaust gases, therefore, by a dramatic increase in the temperature of the exhaust gas, a state that is triggered by introducing an oxidizing agent into the exhaust gas stream. In order to remove the nitric oxides (NOx) from the exhaust gas, a reducing agent has to be injected. This reducing agent can be, for example, a urea/water solution (HWL).
The injection system for such an oxidizing or reducing agent is disposed in the direction of flow at an adequate distance upstream of the catalysts or diesel particulate filters, which are known under the umbrella term as “inserts,” so that before the exhaust gas stream impinges on the insert, the oxidizing or reducing agent mixes adequately well with the exhaust gas. In the ideal case the oxidizing or reducing agent mixes completely with the exhaust gas when it impinges on the insert, so that the oxidizing or reducing agent can flow through the insert in such a way that it is uniformly dispersed over the entire cross section and, hence, can regenerate the insert.
In order to shorten the mixing distance between the injection system and the insert (or rather to achieve a reliable thorough mixing), the prior art discloses the use of turbulizers that are disposed in the flow direction downstream of the injection system. Such turbulizers generate powerful turbulence in the exhaust gas stream; and this turbulence, in turn, dramatically increases the rate at which the exhaust gas and the oxidizing or reducing agent are thoroughly mixed.
However, the turbulence generated by the turbulizer can also generate pressure differentials and backflows of the exhaust gas mixture. They appear predominantly in the region directly upstream of the insert, because the exhaust gas system widens in this region due to the large cross section of the insert. Even with good and thorough mixing of the oxidizing or reducing agent with the exhaust gas, this turbulence may cause a non-uniform dispersion of the exhaust gas stream and, as a result, a non-uniform dispersion of the oxidizing or reducing agent over the cross section, so that the flow through the insert is not uniform. A complete regeneration is thus not guaranteed because of this non-uniform dispersion of the oxidizing or reducing agent.
The object of the present invention is to provide an exhaust gas system that enables a more uniform dispersion of the oxidizing or reducing agent and, thus, a more effective regeneration of the insert.
This and other objects are achieved according to the invention in that an exhaust gas system of the genre described above is provided with a flow rectifier in the exhaust gas conveying duct between the turbulizer and the insert. The invention is based on the concept of stabilizing again the exhaust gas flow after a complete thorough mixing of the oxidizing or reducing agent with the exhaust gas, thus, after an adequately long mixing zone in the exhaust gas conveying duct, in that the turbulence generated by the turbulizer is damped or rather filtered out by way of a flow rectifier. The result of this arrangement is that a uniform flow exhibiting no turbulence, or only low turbulence, is generated upstream of the insert, so that the exhaust gas flow can impinge on the insert, disposed downstream of the flow rectifier, in such a way that it is uniformly dispersed over the entire cross section. As a result, the entire cross section of the insert can be uniformly traversed by flow, so that the entire surface area of the insert can be used for cleaning the exhaust gas, and also an effective regeneration of the whole insert can be guaranteed.
The invention is particularly relevant for use with discontinuous oxidizing or reducing agent injection systems. A complete thorough mixing upstream of the inlet cone of the insert can be carried over to the insert only with a flow rectifier. If a flow rectifier is not used, then the recirculation flows may generate a kind of “de-mixing.” The result of this phenomenon would be that a complete thorough mixing cannot be “carried over” to the insert.
A new generation of turbulence between the flow rectifier and the insert is prevented by the fact that the distance between the flow rectifier and the insert is designed to be as short as possible. For this reason the flow rectifier is disposed preferably directly upstream of the insert.
In order to enable an exhaust gas cleaning that is as effective as possible with negligible flow resistance, the insert has a significantly larger cross section than the exhaust gas conveying duct. Therefore, the exhaust gas conveying duct empties between the turbulizer and the insert into a cone that compensates for this difference in the cross sections. In particular, this cone is disposed between the flow rectifier and the insert. Hence, the exhaust gas conveying duct does not widen until directly upstream of the insert and after the stabilization of the exhaust gas flow. Because of the rectification of the exhaust gas flow by way of the flow rectifier, there is no new formation of a powerful vortex even with the expansion of the cross section of the exhaust gas conveying duct in the cone.
In order to prevent the formation of a new vortex, the cone is disposed directly upstream of the insert, so that the distance between the flow rectifier and the insert can be made as short as possible. Because of the installation conditions, the exhaust gas system is not installed linearly in the vehicle or rather on the underside of the vehicle. That is, the insert can extend with its longitudinal axis obliquely to the longitudinal axis of the exhaust gas conveying duct. This angular difference is leveled out preferably by way of the cone such that the longitudinal axis of the cone usually coincides with the longitudinal axis of the exhaust gas conveying duct, i.e. extends obliquely to the longitudinal axis of the insert. The widened end of the cone is cut off obliquely and adapted to the geometry of the insert.
The flow rectifier can be formed, for example, in a simple way in that at least one wall, in particular, a sheet metal plate, which extends in the longitudinal direction of the exhaust gas conveying duct in the region of the flow rectifier, is provided; and this wall divides the exhaust gas conveying duct into subducts having a smaller cross section. The cross sections of the subducts are designed to be sufficiently small such that a vortex cannot form in them; and/or the propagation of existing turbulence is prevented, so that an effective stabilization of the exhaust gas flow is achieved with this subdivision.
Therefore, the flow rectifier has preferably a plurality of subducts, which extend in the longitudinal direction of the exhaust gas conveying duct. These subducts are so small that they cannot generate a vortex and/or backflows. These subducts are formed, for example, by use of a plurality of walls extending in the longitudinal direction of the exhaust gas conveying duct.
These walls can intersect, when viewed in the longitudinal direction of the exhaust gas conveying duct, so that, when seen in the direction of flow, they form a pattern that resembles a grid.
In this case, the ducts have a polygonal cross section, preferably a hexagonal cross section. Such a hexagonal cross section that resembles a honeycomb offers the advantage that given an ideal usage of the cross section it is possible to provide ducts of identical cross section, i.e. with the same flow resistance.
The turbulizer can be, for example, a static mixer, so that no moving parts are disposed in the exhaust gas conveying duct.
Such a static mixer is, for example, a swirl generator (also known as a swirl turbine), which can generate a strong swirling flow in the exhaust gas system. This swirling flow extends over the entire cross section of the exhaust gas system or, more specifically, the entire cross section of the exhaust gas conveying duct.
An insert is, for example, an SCR catalyst.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.